1. Field of the Invention
This invention relates to a method of making a lithographic printing plate by the silver complex diffusion transfer process. In particular, this invention relates to a method of preventing image degradation of the printing plate due to deterioration of the processing chemistry.
2. Background of the Art
The present invention relates to a single sheet printing plate construction comprising a support, on which an antihalation layer, an emulsion layer and a physical nucleating receptor layer are provided. Following imagewise exposure, the plate is processed in a diffusion transfer developer. The exposed area develops out as a black silver image, while the unexposed silver halide subsequently diffuses through to the receptor element. Physical development nuclei in the receptor layer reduce the silver halide to a metallic silver complex in the presence of the toning agent 1-phenyl-5-mercaptotetrazole.
As described in various embodiments, e.g., U.S. Pat. Nos. 3,728,114, 4,160,670 and U.S. Pat. No. 4,361,635, such a process can function as a lithographic printing plate due to the oleophilic nature of the complexed silver in the receptor layer and the hydrophilic developed silver image in the emulsion layer. However, a very rapid development process is required to reduce exposed areas into non-diffusing silver images before the silver halide solvent can take effect. The greater the differentiation between the (unexposed) complexed silver and the (exposed) developed image area, the better the printing quality will be.
According to James, The Theory of The Photographic Process, 4th. Edition, Macmillan Publishing Co., New York, 1979, page 475, several factors such as silver concentration, particle size and physical development nuclei can affect the color of the transferred silver. Diffusion transfer systems such as monochrome prints, duplicating papers, block copying materials and instant photography require a neutral black, or at least a blue-black, tone. In this respect iodide is well documented as a toning agent or restrainer.
U.S. Pat. No. 3,515,555 describes a direct positive photographic paper with improved tone when processed in a solution containing an alkali metal sulfide, alkali metal hydroxide, thioamide and an alkali metal iodide.
U.S. Pat. No. 3,619,186 describes a diffusion transfer system wherein the receiving layer contains iodide. This results in a neutral tone image. The presence of iodide in the emulsion layer or developer does not have the same beneficial effect on tone.
U.S. Pat. No. 4,124,390 discloses a mordanting solution consisting of an alkaline solution of one or more complexing agents which form an insoluble silver salt complex. Potassium iodide is given as an example of a complexing adjuvant which may also be present.
U.S. Pat. No. 4,220,702 includes an example of a developer for processing a lithographic printing plate that contains potassium iodide as a restrainer for a chloroiodide emulsion. The level of iodide is significantly greater than the amount utilized in an activator for processing an emulsion, such as a chlorobromide emulsion as described in this invention.
U.S. Pat. No. 4,362,811 describes an activator solution for diffusion transfer processing that contains, in addition to the prerequisite alkali, sulfite and silver halide solvent, specific levels of potassium and bromide ions: 2.0-12.0 mole% potassium based upon the total cations of the salts in the activator and 1.7.times.10.sup.-3 to 1.7.times.10.sup.-2 mole/liter bromide.
DT 1,801,330 discloses the combination of 1 -phenyl-5-mercaptotetrazole and potassium iodide in either the developer or image receiving layer of a diffusion transfer process. Such a combination provides black image tone and faster processing time in a peel apart duplicating paper system.
U.S. Pat. No. 4,436,805 also describes the combination of 1-phenyl-5-mercaptotetrazole and potassium iodide to achieve neutral black image tone in a two sheet peel apart diffusion transfer system. Other mercapto or thione heterocyclic nitrogen compounds are claimed in this patent.
U.S. Pat. No. 4,632,896 describes the use of aminoalcohols for processing a two sheet peel apart diffusion transfer system. Examples of solutions containing iodide are given.
U.S. Pat. No. 4,649,096 similarly describes developers for processing two sheet peel apart diffusion transfer systems, wherein examples include potassium iodide.
U.S. Pat. No. 5,037,727, though not a diffusion transfer system, claims the use of iodide in maintaining neutral image tone of activator processed black-and-white photographic prints.